The approaches described in this section could be pursued, but are not necessarily approaches that have been previously conceived or pursued. Therefore, unless otherwise indicated herein, the approaches described in this section are not prior art to the claims in this application and are not admitted to be prior art by inclusion in this section.
The electrophoretic display (EPD) is a non-emissive device based on the electrophoresis phenomenon of charged pigment particles suspended in a solvent. The display usually comprises two plates with electrodes placed opposing each other, separated by using spacers. One of the electrodes is usually transparent. A suspension composed of a colored solvent and charged pigment particles is enclosed between the two plates. When a voltage difference is imposed between the two electrodes, the pigment particles migrate to one side and then either the color of the pigment particles or the color of the solvent can be seen according to the polarity of the voltage difference.
There are several different types of EPDs, such as the conventional type EPD, or the microcapsule-based EPD or EPD with electrophoretic cells that are formed from parallel line reservoirs. EPDs comprising closed cells formed from microcups filled with an electrophoretic fluid and sealed with a polymeric sealing layer are disclosed U.S. Pat. No. 6,930,818, the content of which is incorporated herein by reference in its entirety for all purposes as if fully disclosed herein.
An EPD may be driven by a uni-polar or bi-polar approach. Under a uni-polar approach, the pixels in a display device are driven to their destined states in two consecutive driving phases. In phase one, selected pixels are driven to a first color state and in phase two, the remaining pixels are driven to a second color state that contrasts with the first color state. For example, in phase one, selected pixels may be driven to a first display state in which the charged pigment particles in the dispersion layer are at or near the non-viewing side of the display device, and in phase two, the remaining pixels are then driven to a second display state in which the charged pigment particles are at or near the viewing side of the display device. Alternatively, the charged pigment particles of selected pixels may first be driven to at or near the viewing side of the display device and the charged pigment particles of the remaining pixels may then be driven to at or near the non-viewing side.
Under a bipolar approach, a driving biasing voltage of a first polarity drives selected pixels to a first display state, and a second biasing voltage of the opposite polarity drives the remaining pixels at the same time to a second state to form a display pattern or image. For example, a positive bias voltage may be applied to the pixels so that a state in which the charged pigment particles are at or near the viewing side of the display device is reached and a negative bias voltage is simultaneously applied to the remaining pixels so that the charged pigment particles are at or near the non-viewing side of the display device.
The bipolar approach tends to be faster than the uni-polar approach, but the electronic drivers used in the bipolar approach tend to be more costly because of their bipolar nature. In either case, the selection of detailed waveform characteristics for driving electrophoretic displays is based on a number of features. For example, if the DC balance, or average voltage applied across the display material is zero when integrated over a substantial time period, the display contrast may be improved and ghost images may be reduced. In addition, if the color or display state of a pixel remains unchanged in consecutive images, the charged pigment particles in that pixel do not get refreshed during transition between the images. As a result, the image uniformity may deteriorate after a period of time, if the display does not have good bi-stability (i.e., maintaining images without power). Driving methods for a bistable display thus must be carefully selected to deal with these phenomena.